1. Technical Field
The present invention relates to an ink-circulation type inkjet printing apparatus.
2. Related Art
An ink-circulation type inkjet printing apparatus for printing in which ink is discharged from an inkjet head while ink is circulated is known.
The ink-circulation type inkjet printing apparatus includes the one in which a pressurizing tank is arranged at a high position and a negative-pressure tank is arranged at a lower position with respect to the inkjet head (see Patent Literature 1, for example).
In this type of inkjet printing apparatus, supply of ink from the pressurizing tank to the ink jet head and recovery of the ink from the inkjet head to the negative-pressure tank are performed by a water head pressure based on a positional relation between the pressurizing tank, the negative-pressure tank, and the inkjet head. To the pressurizing tank, the ink is fed from the negative-pressure tank by an ink pump.
In the type of inkjet printing apparatus as described above, in a standby state in which the ink circulation or printing is not performed, the negative-pressure tank is open to the atmospheric air. On the other hand, the pressurizing tank is brought into a sealed state by a valve in order to prevent the ink from flowing from the pressurizing tank to the negative-pressure tank.
In the inkjet head in which nozzles are arranged with high density in order to make printing with high resolution possible, an ink channel in the inkjet head is narrow, and channel resistance is large. When this type of the inkjet head is to be used, a required ink flowrate cannot be ensured only by the water head pressure by arrangement of the inkjet head, the pressurizing tank, and the negative-pressure tank as described above in some cases.
Thus, in order to ensure the required ink flowrate, an inkjet printing apparatus is known in which a positive pressure is applied to the pressurizing tank and a negative pressure is applied to the negative-pressure tank by using an air pump. In this type of the inkjet printing apparatus, the pressurizing tank and the negative-pressure tank are brought into the sealed state by valves, respectively, in the ink circulation, and a pressure is applied by an air pump. In the standby state, the pressurizing tank is brought into the sealed state, while the negative-pressure tank is brought into a state open to the atmospheric air as described above.
In this type of the inkjet printing apparatus, a normally-closed type electromagnetic valve is used which is open when being electrified and is closed when being non-electrified as a valve for switching the pressurizing tank between the state open to the atmospheric air and the sealed state. On the other hand, a normally-open type electromagnetic valve is used which is closed when being electrified and is open when being non-electrified as a valve for switching the negative-pressure tank between the state open to the atmospheric air and the sealed state. As a result, in a non-electrified state of each valve in the standby state or in a power-off state, the pressurizing tank can be brought into the sealed state, while the negative-pressure tank can be brought into the state open to the atmospheric air.
Here, the pressurizing tank is in the sealed state both during the ink circulation and standby, but when the ink circulation is finished and the state proceeds to the standby state, the valve of the pressurizing tank is temporarily opened so as to open the pressurizing tank to the atmospheric air and to release the pressure. That is because, if the negative-pressure tank is opened to the atmospheric air while the pressurizing tank is sealed and kept in a positive pressure, a pressure applied to the nozzle in the inkjet head rises and breaks meniscus of the ink, and there is a concern of ink leakage.